Plug comparators for the bench checking of holes of mechanical parts are already known.
For example, in Italian Patent 1,010,553 there is described a plug comparator for the manual checking of holes which comprises two movable arms bearing at one end two feeler elements and at the other end two mutually movable parts of a transducer. The movable arms are fastened at intermediate points to an element fastened to the frame of the comparator comprising two thin flexible sections which act as fulcrums. The comparator furthermore has a casing for the centering of the comparator in the hole and for the protection of the movable arms and of the feeler elements.
Important requirements of a comparator, in particular of a bench comparator, are, in addition to the precision and the reproducibility of the measurements, low cost of production, maintenance and repair, strength, and the possibility of adapting the comparator to different measurement ranges.